1. Field of the Invention
The present invention relates to a semiconductor type pressure sensor and more particularly to a pressure sensor for the measurement of high-pressure liquids and gases.
2. Description of the Prior Art
As an example of the prior art relating to the present invention, mention is made of Japanese Patent Laid-Open No. 5-273063. The sensor disclosed in this unexamined publication mainly comprises, as shown in FIG. 1 thereof, a stem having through holes for the introduction of pressure, a glass substrate also having through holes and bonded to the stem by welding for example in a communicated state of its through holes with the through holes of the stem, and a pressure sensing silicon element bonded to the outer peripheries of the through holes by anode coupling for example. When pressure is introduced from the pressure introducing through holes, a diaphragm portion formed in the pressure sensing element is deformed, the resistance value of a strain gauge formed on the diaphragm changes, and an electric signal proportional to the pressure is outputted. However, this structure involves the problem that breakage is apt to occur due to, for example, peeling-off of the bonded portion upon application of a high pressure because tensile stresses are imposed on various portions under the application of pressure.
An example of a semiconductor type pressure sensor having a structure for high pressures is shown in Japanese Patent Laid Open No. 2-138776. In this semiconductor type pressure sensor, as illustrated in FIG. 1 thereof, a pressure sensing unit comprising a silicon chip and a glass substrate is disposed in a spatial portion of a cylindrical, metallic pressure introducing pipe formed in a housing, and the other portion thereof than diaphragm portion is hermetically sealed in the said pipe with a low-melting glass.
Thus, also in the structure of Japanese Patent Laid Open No. 2-138776, it is only a low-melting glass that is used for keeping the interior of the sensor in a hermetically sealed state against a medium to be measured. Therefore, in the event the low-melting glass is cracked under the application of a high pressure thereto, it is very likely that the medium to be measured will get into the sensor. This is very dangerous if the medium to be measured is a combustible substance such as, for example, gasoline.
The structure of the conventional pressure sensing unit involves a further problem such that in designing a diaphragm in consideration of pressure resistance and output characteristics, an optimum shape thereof differs depending on the pressure range to be measured and that therefore a pressure sensor designed for high pressure is difficult to ensure a high accuracy for a low pressure range.